In X-Menays of future Past there Isn't going to be a whole let of openings for new
Characters.You have some time In future where actors from original Trilogy will be used.
In First Class era there Isn't going to be a lot of time to Introduce new characters.The one new character to go toe to toe with Magneto Vaughn mentioned could be character from future(and could be Wolverine) so Inless they start the timeframe of first Class era with a new X-Man already on team there Isn't going to be lot of time spent Introducing new X-man plus future scenes of original trilogy actors.Now they could always drop Angel and Riptide from Brotherhood In favor of a different henchman(I don't see Vaughn dropping Azarel since he puts The acotr In all his films) for Magneto with Mystique and Emma Frost.